This proposal requests continued support of a broadly based research program that seeks to understand (i) the evolution of hormone-brain-behavior mechanisms underlying sexual behaviors, and (ii) the dual neural circuits that subserve these behaviors. This goal is relevant to mental health since information on the brain mechanisms that control normal behavior provides the context in which to determine and evaluate psychopathology. The approach used is both comparative and multidiscipline. 'Me comparative method emphasizes the need for different model systems if we are to elucidate both the general rules which govern behaviors as when as their historical roots. Multidisciplinary studies spanning the molecular to the population levels of biological organization provide a multifaceted, yet integrated, perspective of individual behavior. The animal model systems to be used are parthenogenetic whiptail lizards and their sexual ancestors. The unisexual species is known to have descended directly from extant sexual species, thereby allowing direct ancestor-descendant comparisons. Because all individuals have ovaries, yet exhibit both male-like and female-like pseudosexual behaviors, the complication of having two gonadal sexes, each with their own particular hormonal milieu, is removed. This makes it possible to study the neural circuits that underlie mounting and receptive behavior in a manner not possible with more common laboratory animals. The studies proposed center on a comparison of the sexual ancestral species and their parthenogenetic descendants. They employ behavioral observation in both the field and the laboratory, functional neurology of specific behaviors as assessed by hormonal manipulation including intracranial implantation, hormone-receptor analysis, radioimmunoassay, immunocytochemistry, and autoradiography. Some experiments will exploit the fact that progesterone stimulates, rather than inhibits, the sexual behavior of males of the parental species and that progesterone activates male-like pseudosexual behavior in the unisexual descendant species. These studies will examine the mechanism of hormone action at the level of the receptor. Other studies will localize the sites of hormone action in the brain, show how different sexual behaviors are reflected in dimorphisms in brain nuclei, and identify and describe the dual neural circuits controlling mounting and receptive behavior.